It's not possible that you would get a Trial version elapsed message after 15 minutes.
The Trial last for 5-6 minutes so, either you see it after 5-6 minutes, because your scenery is not activated, or you will not get it.
If you have the Trial elapsed message after 5-6 minutes, which is the only possible time you could ever see it, it's because you haven't activated the scenery so, the first thing to check is the Addon Manager, which will tell you if the scenery is Active or in Trial.
If it's still in Trial, you must activate it with the "Register Serial" button.